(1) Field of the Invention
The present invention relates to stripline microwave applicators particularly for creation and maintenance of mini and micro microwave (plasma) discharges. The apparatus and methods described are directed toward efficiently creating and precisely controlling very small microwave discharges (plasmas). These discharges have typical physical dimensions, d, that are less than a millimeter and as small as a few tens of microns. The free space wavelength, xcex, of microwave energy (300 MHz-30 GHz) varies from one meter to one centimeter and thus xcex is much greater than d throughout the entire microwave frequency spectrum. In particular, the present invention relates to an apparatus wherein the stripline conductors that couple microwave energy are transverse to the microwave discharge, and preferably with a container for generating the plasma so that the plasma extends beyond the stripline excitation region.
(2) Description of Related Art
It is also well known that a condition for the existence of a plasma discharge is that d greater than (6-10)xcexDE 
where             λ      DE        =          743      ⁢              xe2x80x83            ⁢                                    T            e                                n            e                              ⁢      Cm        ,
Te is the electron temperature in volts, and ne is the electron density in electrons per cm3. This criteria implies that to produce very small plasmas (discharges) high densities and low electron temperatures are desirable. For example to create 100 micron size microwave plasmas the Debye length, xcexDE, must be approximately 10-15 microns. If Texcx9c4 volts then ne≳1012 cmxe2x88x923. If dxcx9c10 microns and if Texcx9c1 volts, then ne≳1014 cmxe2x88x923. Thus very small discharges require low electron energies and high charge densities, and are as a result very intense discharges that have very high absorbed power densities (W/cm3). Despite the required high power densities the total absorbed power of these discharges is very low, i.e. of the order of a few watts or less.
The high density ne requirement of these very small microwave plasmas implies that ne greater than  greater than nc where nc is the critical density. The critical density nc is defined as the density where, f, the excitation frequency is equal to the plasma frequency, fpe. That is when
f=fpe=8980{square root over (nc)}Hz
where nc is in units of cmxe2x88x923. Very small plasmas require very high electron densities, ne. Thus ne greater than  greater than nc. Therefore, the microwave plasma will be over dense, and as a result the electromagnetic energy will not freely propagate through the discharge, but will exist in a thin discharge surface layer equal to about the skin depth, xcex4c, where       δ    c    =                    (                              2            ⁢                          mν              eff                                            ω            ⁢                          xe2x80x83                        ⁢                          μ              o                        ⁢                          e              2                        ⁢                          n              e                                      )                    1        /        2              =                  c                  ω          pe                    ⁢                        (                                    2              ⁢                              ν                eff                                      ω                    )                          1          /          2                    
However, for very small discharges xcex4c greater than d.
This condition indicates that higher excitation frequencies will more readily produce higher density discharges. The required high densities also impose conditions on discharge pressure. To readily achieve the required high densities it is desirable to operate these discharges at moderate pressures (≳Torr) to higher pressure environments (one or more atmospheres) where high species densities are available and where xcexdeff/xcfx89 is greater than one thereby insuring some electromagnetic energy penetration within the discharge.
Mechanisms of coupling energy into microwave discharges vary with pressure. At low pressure the effective electron collision frequency, xcexdeff, is much less than xcfx89. Thus energy coupling takes place primarily via stochastic heating and resonant wave/collisionless heating mechanisms. These mechanisms include such phenomena as electrons impinging on the oscillating sheath edge and wave particle interactions that occur in electroacoustic wave/surface wave plasma interactions. As the pressure is increased xcexdeff increases and thus electromagnetic/discharge coupling takes place via an electron collisional process, i.e. ohmic heating.
Early microwave discharge experiments demonstrated the formation of relatively small discharges with dimensions of a few mm or larger and with microwave absorbed power levels of a few Watts or more (Fritz, R., M. S. Thesis, Michigan State University, East Lansing, Mich. (1978); and Asmussen, J., Thesis, University of Wisconsin (1967); Asmussen, J., et al., Appl. Phys. Letters 11, 324-326 (1967); Asmussen, J., et al., IEEE Trans. Electron Devices, ED-16, 19-29 (1969)). During the period of these experimental investigations it was envisioned that the practical application of microwave discharges required discharges with typical dimensions of several centimeters or more. Thus research efforts were directed toward development of applicator coupling techniques that created and maintained large volume, high density discharges with dimensions of 8.0-40 cm. These efforts resulted in a variety of microwave discharge configurations such as those described in Asmussen, J., et al., IEEE Trans. In Plasma Science, PS-25, 1196-1221 (1997); and Popov, G., High Density Plasma Sources, Chapter 6, Noyes Pub. (1996)) and in U.S. Pat. Nos. 4,507,588; 4,585,688; 4,630,566; 4,727,293 and 5,081,398 to Asmussen. 
Using this technology with 2.45 GHz excitation, large volume discharges were created strategically locating the bounded plasma volume within the applicator. The optimal location of the discharge volume allowed the discharge to be exposed to a relatively large region (in comparison to the excitation wavelength) of applied electromagnetic field. Additionally the applicator had to be adjustable to enable first the ignition of the discharge and then the efficient matching of high power (100-thousands of watts) into the high density plasma. Then these applicator/discharge configurations were scaled up by decreasing the excitation frequency to 915 MHz. These techniques were successful in creating uniform microwave plasmas over a pressure regime of a few millimeters to over 200 Torr with dimensions of 10-35 cm.
However, the applicator technologies that were developed to create large discharges, are not optimal for the formation of small discharges. If the excitation frequency is raised the waveguide and cavity applicators become smaller and thus become more difficult to fabricate.
One method of producing high density discharges is by the use of rf inductive plasma coupling via planar or helical coils (Lieberman, M. A., et al., xe2x80x9cPrinciples of Plasma Discharges and Materials Processing,xe2x80x9d John Wiley and Sons, (1994)). Inductive coupling results in the noncapacitive power transfer to the charged species of this discharge, thereby achieving a low impressed voltage across all plasma sheaths at electrode and wall surfaces. These high density plasma sources are typically excited by 13.56 MHz rf energy and are capable of producing large 10-40 cm diameter discharges with densities in excess of 1012 cmxe2x88x923. Thus ne greater than  greater than nc and xcex greater than  greater than d and their behavior can be understood by quasistatic electromagnetic analysis. These discharges represent an important method of electromagnetic/plasma excitation, i.e. quasistatic inductive excitation.
Recently, Hopwood et al has scaled these inductive planar discharges down to very small dimensions (Yin, Y., et al., xe2x80x9cMiniaturization of Inductively Coupled Plasma Sources,xe2x80x9d IEEE Trans. Plasma Science, 27, 1516-1524 (1999)). See also U.S. Pat. No. 5,942,855 to Hopwood for a small plasma generator. Small planar coils of 5-15 mm diameter were fabricated and were excited with 100-460 MHz rf energy. These small discharges demonstrated the ability of inductive coupling at high frequencies to sustain small high density plasmas. Microfabrication techniques were used to fabricate the small planar inductive coils. However, these experiments indicated that as rf frequency was increased the coupling efficiency decreased. Small plasmas required high plasma densities which in turn require high excitation frequencies (xcx9c1-5 GHz) and the fabrication of smaller inductive coils that must operate at higher and higher current and power densities. It was suggested that the power density and coupling efficiency will prevent the application of this quasistatic excitation method to plasmas smaller than xc2xd-1 mm.
Another microwave applicator that is capable of producing very small microwave discharges is the coaxial applicator shown in FIG. 1. The applicator consists of an outer conductor with inner diameter of 2.2 cm and a center conductor with a diameter of approximately 1 cm. As shown, the discharge is ignited and sustained in a break or gap in the center conductor. The capacitive gap of approximately 1-5 cm is filled with a plasma and thus this type of discharge is often referred to as a plasma capacitor (Lee, Q. H., xe2x80x9cAn Experimental Study of Nonlinear Phenomena in a Resonantly Sustained Microwave Plasma,xe2x80x9d Ph.D. Thesis, Michigan State University (1970); Asmussen, Ph.D. Thesis, University of Wisconsin (1967); J. Asmussen and J. B. Beyer, Appl. Phys. Letters, 11, 324-326 (1967); J. Asmussen and J. B. Beyer, IEEE Trans. Electron Devices, ED-16, 19-29 (1969)). Small, 1 cm diameter by 1-2 mm, high density (1011-1012 cmxe2x88x923) plasma capacitive discharges have been created by this applicator. While this applicator has the ability to create very small discharges it is unlikely that the coaxial applicator can be scaled down to dimensions that enable its fabrication on a chip.
In 1931, Tonks (Phys. Rev. 37, 1458 (1931); Phys. Rev. 38, 1212 (1931)) observed the phenomenon called plasma resonance oscillations, in a bounded uniform plasma when the plasma frequency xcfx89pe is greater than the excitation frequency. Since that time this oscillation was observed in many experiments (Parker, J. V., et al., Phys. Of Fluid, 7, 1489 (1964); Phys. Rev. Letters, 11.183 (1963); and Taillet, J., Am J. Phys., 37, 423 (1969)) and has been identified as a space charge oscillation in a bounded plasma, and is now identified as a xe2x80x9ccold plasma resonance.xe2x80x9d In 1951 Romell (Romell, D., Nature, 167, 243 (1951)) observed that a cylindrical plasma discharge when subjected to microwave scattering exhibited a main cold plasma resonance and a series of weaker resonances which are not predicted by the uniform, cold plasma model of Tonks. Since then these additional dipolar resonances have often been referred to as xe2x80x9cTonks-Dattnerxe2x80x9d or xe2x80x9cT-Dxe2x80x9d resonances.
Many years later the observed cold plasma and T-D resonance spectrum was finally theoretically explained with the use of a plasma theory that included the thermal motion of the plasma electron gas, and allowed the existence of electron plasma waves. Additionally the bounded plasma nonuniformity, i.e. the plasma density profile influenced the exact location of these resonances. Good agreement between theory and experiment was achieved (Parker, J. V., et al., Phys. Of Fluid, 7, 1489 (1964); Phys. Rev. Letters, 11.183 (1963)) when a plasma density profile corresponding to the Tonks-Langmuir (Tonks, L., et al., Phys. Rev., 34, 876 (1929)) model was included. The calculated resonances showed excellent quantitative agreement with experiments for the main (cold plasma) and the first two temperature resonances. More recently, W. M. Leavens (Leavens, W. M., Radio Science, 69D, 10, (1964) 1321; Phys. Fluid, 10, 2708 (1967)) and D. E. Baldwin (Phys. Fluid, 12, 279 (1969)) developed a kinetic model for the temperature resonances. In both cases, Landau damping (collisionless damping), which is present near the tube wall is included in the analysis. Experimental confirmation was made without choosing the electron temperature for the best fit.
During the 1970-1980 period a number of investigators demonstrated that one could efficiently couple microwave energy into and sustain a discharge if this energy was coupled into these plasma resonances. In fact if enough power was available a microwave discharge could be sustained at the cold plasma resonance or at the first or second T-D resonance. These microwave discharges were identified as resonantly sustained discharges. Microwave discharges were formed in waveguide (Lee, Q. H., xe2x80x9cAn Experimental Study of Nonlinear Phenomena in a Resonantly Sustained microwave Plasma,xe2x80x9d Ph.D. Thesis, Michigan State University (1970)) and cylindrical coaxial cavity applicators (Fredericks, R. M., et al., xe2x80x9cRetuning and Hysterisis effects of a rf plasma in a variable size microwave cavity,xe2x80x9d Appl. Phys. 42, 3647-3649 (1971); Fredericks, R. M., et al., xe2x80x9cA High density resonantly sustained plasma in a variable length cylindrical cavity,xe2x80x9d Appl. Phys. Letters, 19, 508-510 (1971); Asmussen, J., et al., Proc. IEEE 62, 109 (1974); and Asmussen, J., Ph.D. Thesis, University of Wisconsin (1967); J. Asmussen and J. B. Beyer, Appl. Phys. Letters, 11, 324-326 (1967); J. Asmussen and J. B. Beyer, IEEE Trans. Electron Devices, ED-16, 19-29 (1969)) and were maintained inside these applicators via coupling to the electromagnetic resonances of the plasma loaded applicator. When the experimental conditions were appropriately adjusted microwave discharges were created by coupling either to the cold plasma resonances of the discharge geometry or to the xe2x80x9cT-Dxe2x80x9d traveling wave resonances (Rogers, J., and J. Asmussen IEEE Trans on Plasma Science PS-10, 11-16 (1980); Fredericks, R. M., Ph.D. Thesis, MSU (1971); and Fritz, R., M. S. Thesis, Michigan State University (1978)).
Bilgic et al (Plasma Sources Sci. Technol. 9, 1-4 (2000)) were the first to describe a stripline applicator for producing a plasma and applied it to atomic emission spectrometry. This research is also evidenced in DE19851628. In this application the stripline applicator is parallel to the container. This particular microwave stripline system couples microwave power into a plasma loaded applicator resonance.
Objects
It is an object of the present invention to provide improved stripline applicators for generating a plasma discharge. It is particularly an object of the present invention to provide applications which are inexpensive to manufacture and which operate effectively. These and other objects will become increasingly apparent by reference to the following description.
This invention has several unique features beyond the prior art. First, it employs microwave stripline applicator technology to create and maintain discharges/plasmas. However the microwave applicator coupling technology described herein is fundamentally different from that recently described by Bilgic et al.
Bilgic et al describes a plasma loaded applicator where the plasma discharge and the stripline coupling structure form an interdependent microwave resonant circuit. The plasma discharge is created and maintained physically inside the stripline applicator, and the stripline electromagnetic fields are impressed over the entire discharge volume, i.e. applicator electromagnetic excitation occurs over the entire discharge. Thus this stripline applicator coupling method is similar to earlier developed, nonstripline applicators and therefore has some of the same fundamental limitations such as limited discharge variability, stability problems, difficulty in matching, and the need for variable tuning. The discharge is located only inside the applicator and thus the discharge size is also limited to the applicator size.
The Bilgic apparatus limits the plasma size to the stripline applicator. Optimal coupling to the discharge loaded stripline applicator occurs when the plasma loaded stripline applicator""s impedance matches or closely matches the input transmission line characteristic impedance. This usually occurs at or near a plasma loaded applicator resonance and often also requires additional external stripline matching stubs for versatile operation. Since the plasma loaded applicator resonance is dependent on the plasma characteristics, such as the average density, the density profile, the effective electron collision frequency, etc., the discharge matching and the discharge stability are very sensitive to changes in external operating conditions such as variations in pressure, input power, gas flow, gas type, and even slight changes in excitation frequency. Some of these limitations can be overcome by adding the appropriate variable tuning as has been utilized in earlier nonstripline applicator designs (See U.S. Pat. Nos. 4,507,588; 4,585,688; 4,630,566; 4,727,293 and 5,081,398 to Asmussen). However, variable tuning may be difficult to achieve and thus may be impractical in microwave stripline applicators.
Unique features of this invention are the microwave coupling to a plasma resonance, the ability to produce stable and matched discharges, and the ability to create discharges external to the microwave coupling region. Coupling to a discharge plasma resonance is excitation frequency insensitive. Thus discharges can be created and maintained with a variety of stripline applicators, which vary from unmatched, nonresonant, stripline circuits to perfectly matched plasma loaded resonant circuits. In all cases the microwave excitation zone occurs in a relatively localized coupling region of the applicator. Microwave energy is coupled into the discharge via a plasma resonance that can be (1) a localized plasma geometric resonance, i.e. a plasma space charge oscillation at the discharge geometric resonant frequency, and (2) either a plasma standing wave or a traveling wave that exists along the discharge container. In the later case the plasma volume increases with an increase in microwave power to a size that far exceeds the applicator excitation region. Then the discharge occupies a volume that is mostly outside the stripline applicator excitation zone. Thus the excitation of standing and traveling waves allows the formation of discharges on curved and multiple channel discharge containers.
The coupling to a plasma resonance produces a stable, matched discharge that is able to be maintained continuously as pressure, gas mixture and flow rate and input power are all varied over a wide range. For example if sufficient power is available, the discharge can be sustained from a few mTorr to over one atmosphere. The flow rate can be varied from no flow to 1000""s sccm. Under certain operating conditions it may be desirable to add external impedance matching to the microwave stripline circuit, but it usually is not absolutely necessary. The resulting microwave coupling system is operationally robust and versatile i.e. it is energy efficient, stable and adaptable to wide variations in operating conditions, and can be scaled to small dimensions.
One of the unique aspects of this disclosure is the microwave electric fields are used to couple the microwave energy into the discharge plasma""s natural space charge oscillations or electron plasma oscillations. Because of the small size of the applicator the coupling can be understood as a quasistatic coupling, i.e., like a resonant plasma capacitor. These natural resonant frequencies that the stripline applicator excites will generally be the natural resonant frequencies of cylindrical or spherical plasmas.
The coupling in these stripline applicators can be to either standing waves or traveling waves where the plasma frequency is greater than the excitation frequency, i.e. for high density plasmas, i.e. the plasma density is greater than the critical density.
A key concept is that the stripline applicator is used to couple to plasma resonances. The plasma resonance can be either a stationary standing wave or traveling wave. In the case of traveling plasma waves the plasma can grow to a size far exceeding the applicators high electric field excitation region.
The method of coupling microwave energy into the discharge that is employed in this invention is coupling via a plasma resonance that is dependent on the geometry of the discharge. For example, common discharge geometries are spherical, cylindrical and even parallel plate plasma slabs. Again, in this type of electromagnetic/plasma coupling the excitation electromagnetic wavelength, xcex, is much larger than the typical physical length, d, of the discharge, i.e., xcex greater than  greater than d. The electromagnetic field creates the discharge by exciting a geometric plasma resonance. This plasma resonance involves exciting inductive space charge oscillations or electron plasma oscillations within the discharge volume. These inductive plasma oscillations then resonate with the capacitive fields that exist in the surrounding exterior of the discharge.
The discharge is held in an ionized state where the electron density, ne, is higher than the critical density nc, i.e. ne greater than  greater than nc and xcfx89pe greater than  greater than xcfx89 where xcfx89 is 2xcfx80 times the excitation frequency. When at resonance the discharge electron/ion densities are related to the geometry of the discharge. For example for a long cylindrical rod discharge             (                        ω          pe                ω            )        2    =            2      ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢      for      ⁢              xe2x80x83            ⁢      spherical      ⁢              xe2x80x83            ⁢                        discharge          ⁢                      
                    (                                    ω              pe                        ω                    )                2              =    3.  
When the enclosing cylindrical dielectric constant of the discharge container is included in the calculation then this relationship becomes             (                        ω          pe                ω            )        2    =            (              1        +                  K          eff                    )              1      /      2      
where Keff is the effective dielectric constant of the container. These relationships represent two dimensional geometric resonances. However, if the cylindrical discharge volume is long, then plasma waves can propagate along the axis of the cylinder. Examples of these guided waves are Gould-Trivelpiece modes (cold plasma modes) and electron plasma waves (or sometimes called electroacoustic waves). Each of these modes will have a guided wavelength, xcexg, that depending on the plasma temperature and density, and the cylindrical dimensions, could vary from several millimeters to several centimeters. When the axial length of the cylinder is equal to xcexg/2, xcexg, 3xcexg/2, etc. a three dimensional plasma resonator is created by exciting standing waves along the cylindrical axis of the plasma.
Thus when exciting discharges with this technique cylindrical discharges with very small cross sectional dimensions are created and sustained. Then as more power is coupled they grow axially and fill the cylinder becoming a cylindrical plasma resonator. Example dimensions are cylindrical radii of xc2xd mm to 50 microns while the length is larger than several centimeters. One unique feature of this invention is the direct coupling to a plasma resonance related to the plasma loaded container geometry. This coupling scheme does not require a resonant electromagnetic circuit.
Thus the present invention relates to an apparatus for maintaining microwave plasma discharges which comprises:
a microwave discharge container having an internal section of 1 cm or less in width, which container is positioned in a dielectric material between conductors which serve as a wave guide for the microwaves and as plates for providing an electrical field with an electrically conductive stripline less than about 3 mm thick and less than 2 cm wide providing one of the plates mounted on the container transverse of the cross-section, wherein the plasma is maintained inside the container by a combination of the microwaves and the electrical field in the presence of a gas which forms the plasma which is beyond the width of the stripline.
The container is preferably a tube which has a length which is longer than the width and wherein the tube can extend outside of the dielectric material.
The stripline conductor is preferably a strip of a conductive metal mounted on the dielectric material which is a solid and adjacent the container and wherein another of the conductors mounted on the dielectric material is a ground plate. The conductor ground plate is preferably a strip of metal which has a length which is greater than the width of the container.
The container is preferably a sphere. The dielectric material can be a gas or a solid. Preferably the plasma discharge is excited so that the discharge is maintained by plasma resonance.
The present invention also relates to a method for producing a plasma discharge, the improvement which comprises exciting the discharge in an apparatus which comprises a microwave container having an internal section of 1 cm or less in width, which container is positioned in a dielectric material between conductors which serve as a guide for the microwaves and as plates for providing an electrical field with an electrically conductive stripline less than about 3 mm thick and less than 2 cm wide providing one of the conductor plates transverse of the container cross-section, wherein the plasma is maintained inside the container by microwave energy in the presence of a gas which when ionized forms the plasma in the container.
Optionally the container is placed in a gap of the stripline conductor and the gap length is less than xcex/8.
Optionally one or more discharges are present at power levels less than 100 w for pressures in the container from 0.01 Torr to above one atmosphere.
Optionally the plasma discharge extends beyond the width of the stripline conductor, so that microwave excitation by the direct stripline conductor occupies a small fraction of a discharge volume in the container and wherein optionally there is a gap in the stripline and the gap is less than xcex/8.
Optionally a portion of the container is placed in the electric field created by the stripline conductor.
Optionally where the conductor is shaped and sized to form a resonant element which produces electric fields in all or a portion of the container.
Optionally the container on one or both sides of the stripline conductor divides into two or more container sections or branches so that the plasma discharge fills the two or more container sections or branches.
Optionally the gas can be flowing through the container or stagnant.
Optionally the conductor is shaped and sized to form a resonant element which produces electric fields in all or a portion of the container and where a resonant and matching structure is created by the addition of tuning circuits between the microwave power supply and resonant element.
Optionally the microwaves are supplied directly to the stripline structure without any matching elements.
Optionally the container sections are in curved or bent shapes.
Optionally the stripline conductor is terminated in an electrical or mechanically tunable adjustable load.
Optionally a tunable element is adjusted for plasma discharge ignition and maintenance and the stripline conductor is terminated in an electrically or mechanically adjustable load.
Optionally an amount of power input is used to control a region or length of the container occupied by the discharge.
Optionally there is more than one of the apparatus that are arranged in an array pattern.
Optionally individual discharges are powered from a single source for the microwaves.
Optionally there are more than one of the apparatus, where individual discharges are powered from a single source for the microwaves and where the individual discharges are sustained by coupling from discharge to discharge.
The plasma discharge in the container can be ignited a number of ways. The ignition process is one of first creating some free electrons that can be heated in the applied microwave electric field formed by the stripline applicator. Specific techniques for ignition include providing a high voltage spark to the discharge container, providing a high microwave electric field to the container by applying an initial high input microwave power, and by shining ultraviolet light into the discharge container region.